1. Field of the Invention
The subject invention is directed to apparatus for making steel and, more particularly, apparatus for transferring molten steel from a ladle to a mold.
2. Description of the Prior Art
Various apparatus and processes have been developed for manufacturing steel. In steelmaking operations, it has been found that transferring molten metal to molds presents a step by which slag or other impurities are sometimes introduced. To improve steel quality, various processes for minimizing the introduction of impurities have been developed.
Such processes have included pressure casting processes wherein molten metal is transferred through a pressure tube and into a casting. Briefly, in the pressure casting process a ladle of molten metal is placed upright in an open pressure vessel. A refractory lined lid is then placed over the vessel. One end of the pressure tube is inserted through an opening in the lid and submerged in the molten metal. The opposite end of the tube is then connected to the mold. Air is pumped into the vessel to pressurize it. The air pressure on surface of the molten metal forces the metal upwardly through the pressure tube and into the mold. The metal enters the pressure tube through the submerged end of the tube and flows through the tube and into the mold. Since the molten metal flows from a location under the metal surface near the bottom of the ladle, the process tends to avoid the entrainment of slag in the molten metal and results in a high-quality casting.
In the prior art, pressure tubes have been made of various materials including alumina graphite, zirconia-alumina, high alumina, high alumina tar impregnated and coked, and muddite.
All of these tubes have the disadvantage that their preparation requires final assembly with a metal collar. The collar is bonded to the outside surface of the tube with a castable or mortar. The collar is located adjacent to one end of the pressure tube. The opposite end of the pressure tube is inserted through the lid opening and the pressure tube is passed through the lid opening until the collar engages the pressure vessel lid. The collar is located on the pressure tube such that the collar contacts the pressure vessel lid and one end of the tube is suspended in the molten metal during pressure casting.
To assemble the tube and collar, the metal collar is placed over one end of the refractory tube. The tube is secured to the collar by a mortar or castable that is placed between the inside wall of the collar and the outside wall of the refractory tube. After the tube is thus secured to the collar, a second layer of mortar is applied to the outer surface of the tube adjacent to the innermost end of the collar. This second layer of mortar is intended to prevent leakage of air between the collar and the tube while the tube is under pressurized conditions. Air leaks at this location are particularly undesirable because the air can then become entrained in the steel as it enters the mold. If air reaches the mold cavity, the mold is usually seriously damaged or destroyed. At a minimum, this results in degradation of the steel quality.
In the prior art, air leaks between the collar and the tube were sometimes caused by slippage between the collar and the tube that resulted in cracks in the mortar. Accordingly, various structures were employed to strengthen the engagement between the collar and the pressure tube. For example, in some cases circular grooves were cut in the external surface of the tube so that the castable or mortar could flow into these grooves to better engage the tube. In another example, the tube was provided with a circular groove and a steel retaining ring that was partially received in the groove extended from the tube to provide a circular flange around the tube. This also was found to improve the engagement between the collar and the tube.
Notwithstanding such improved designs, a persistent problem with the use of such collars has been that they potentially allowed passage of air through mortar cracks or seams between the pressure tube and the collar. This also created a potential for air to become entrained in the steel and carried into the mold. Moreover, the prior art process for assembling collars to the refractory tubes required substantial labor, time and space to complete. All of these requirements significantly added to the overall cost of the pressure casting process.
Thus, there was a need in the prior art for an improved design for pressure tubes that would further reduce the likelihood that a pathway between the collar and the refractory tube would develop and entrained air would enter the mold. Preferably, an improved design could also substantially reduce requirements for time, labor and space that were associated with the collar assembly process.